The use of an internal gear type rotary pump such as a trochoid pump or the like as a pump for a braking device has been conventionally proposed (e.g., see Japanese Patent Laid-Open Publication No. 2004-11520).
FIGS. 5A, 5B, and 5C are cross-sectional views of a conventional rotary pump for a braking device. FIG. 5A is a cross-sectional view of the rotary pump that is cut along a plane perpendicular to its rotating axis. FIG. 5B is a cross-sectional view taken along a line A-A in FIG. 5A. FIG. 5C is a cross-sectional view taken along a line B-B in FIG. 5B.
The rotary pump shown in FIGS. 5A, 5B, and 5C is composed of an outer rotor 51 having an inner toothed portion 51a along an inner periphery thereof, an inner rotor 52 having an outer toothed portion 52a along an outer periphery thereof, a casing 50 for accommodating the outer rotor 51 and the inner rotor 52, and the like. The outer rotor 51 and the inner rotor 52 are disposed in the casing 50 with the inner toothed portion 51a and the outer toothed portion 52a meshing with each other to form a plurality of interstitial portions 53.
The casing 50 is composed of a first side plate 71 disposed on an end face of the outer rotor 51, a second side plate 72 disposed on an end face of the inner rotor 52, and a central plate 73 in which a circular hole corresponding in shape to the outer rotor 51 is formed. The outer rotor 51 and the inner rotor 52 are accommodated in a rotary chamber formed by the casing 50.
Given that a line passing through both a central axis X of the outer rotor 51 and a central axis Y of the inner rotor 52 is a centerline Z of the pump, a suction port 60 and a discharge port 61 communicating with the interstitial portions 53 are provided on opposed sides with respect to the centerline Z. When the pump is driven, the inner rotor 52 makes a rotational movement via a drive shaft 54 whose center coincides with the central axis Y of the inner rotor 52. In accordance with the rotational movement of the inner rotor 52, the outer rotor 51 also rotates in the same direction due to the meshing of the inner toothed portion 51a with the outer toothed portion 52a. In this case, the respective interstitial portions 53 increase and decrease in volume while the outer rotor 51 and the inner rotor 52 rotate by 360°. Thus, brake fluid is sucked from the suction port 60 and discharged from the discharge port 61.
One axial end face of the rotary pump is sealed by a sealing member 100 equipped with an elastic member 100a such as rubber and a resin member 100b. More specifically, the sealing member 100 is accommodated in a sealing groove portion 71b formed in a side seal 71, and performs a sealing function when the elastic member 100a presses the resin member 100b. 
The other axial end face of the rotary pump is mechanically sealed by bringing end faces of the inner rotor 52 and the outer rotor 51 into direct contact with the second side plate 72. The outer rotor 51 and the inner rotor 52 are pressed toward the second side plate 72 by the resin member disposed on said one face and a brake fluid pressure. This pressing force brings the outer rotor 51 and the inner rotor 52 into contact with the second side plate 72. In this manner, the above-mentioned mechanical sealing is realized.
In the case where one axial end face of the rotary pump is thus mechanically sealed, the friction resistances between the outer rotor 51 and the inner rotor 52 on the one hand and the second side plate 72 on the other increase on the side of that end face, thus causing an increase in driving torque. Also, the contact areas between the outer rotor 51 and the inner rotor 52 on the one hand and the second side plate 72 on the other increase. Therefore, especially at a low temperature corresponding to a high viscosity of brake fluid, the shear resistance of brake fluid increases and thus causes an increase in driving torque.
Thus, in the conventional rotary pump, a suction groove 72d communicating with the suction port 60 and a discharge groove 72e communicating with the discharge port 61 are provided in the second side plate 72 on the mechanically sealed side.
The suction groove 72d and the discharge groove 72e serve to introduce fluid pressures into the suction port 60 and the discharge port 61, press back the outer rotor 51 and the inner rotor 52, and decrease a force for pressing the outer rotor 51 and the inner rotor 52 to the second side plate 72, thus achieving a reduction in friction resistance. This makes it possible to prevent an increase in driving torque.
In the aforementioned conventional rotary pump, as shown in FIG. 5C, the curvature of a circular arc constituted by a side 72ea of the discharge groove 72e on the side of the drive shaft 54 is set equal to that of a circular arc constituted by a side 72da of the suction groove 72d facing the side 72ea of the discharge groove 72e. That is, an illustration in FIG. 6 is obtained by partially enlarging the discharge groove 72e and the suction groove 72d in FIG. 5C. Thus, centers O1 and O2 of the circular arcs constituted by the sides 72da and 72ea respectively are identical to each other. Accordingly, when the suction groove 72d and the sealing groove portion 71b are viewed in the axial direction of the drive shaft 54, the distance between those sides is basically constant.
The suction groove 72d is axisymmetrical with respect to the line W perpendicular to the centerline Z passing through the central axis Y. From the side facing the discharge groove 72e to a region communicating with the suction port 60, the suction groove 72d is formed as an inclined straight line portion 72db and has a gradually increased groove width in a direction parallel to the centerline Z.
By conducting an endurance test as to the rotary pump having the suction groove 72d and the discharge groove 72e formed as described above, the inventors have confirmed contact traces on the second side plate 72 in a region from the straight line portion 72db of the suction groove 72d to the discharge groove 72e. These contact traces are considered to be ascribable to a partial increase in contact surface pressure and thus in contact resistance between the second side plate 72 on the one hand and the outer rotor 51 and the inner rotor 52 on the other. This increase in contact resistance is considered to be the cause of an increase in driving torque of the rotary pump.